Add IBM Watson Speech to Text provider

[katlark]

Summary

• add an IBM Watson Speech to Text provider using the synchronous /v1/recognize HTTP API
• expose it through sapat --api ibm
• document the required IBM Watson environment variables in the README
• add focused mocked unit coverage for endpoint construction, transcript extraction, missing config, and CLI routing

Validation

• .venv/bin/python -m unittest discover -s tests -v
• .venv/bin/python -m py_compile src/sapat/script.py src/sapat/transcription/watson.py tests/test_watson_transcription.py tests/test_script.py
• .venv/bin/python -m sapat.script --help
• git diff --check

Companion implementation for Daytona content bounty daytonaio/content#13.

No API keys, recordings, payment details, private account data, or local sensitive paths are included.

[katlark]

Closing as duplicate/overlapping with #27, which opened the same IBM Watson provider slice first. I am pivoting the Daytona bounty work to a distinct Together AI provider instead.